Hyperthermia, raising the temperature of tissue by the application of heat, has proven to be useful of the treatment of tumors particularly in combination with radiation therapy or chemotherapy. As a result the need has arisen for practical methods of providing localized heating in arbitrary tissue volumes. Heating superficial tumors in hot water baths has been proposed, but the method has very limited clinical application. Microwave and ultrasound diathermy are capable of greater penetration than external conduction heating, but are also generally limited to the heating of superficial tissue volumes.
In the contrast with the prior art limitation to essentially superficial tissue heating, the present invention enables localized tissue heating by electric current fields produced by specified electrode configurations of deeply embedded tumors virtually anywhere in the body. Heat may be applied in almost any treatment volumes specified by the therapist. Variations in electrical and thermal properties of tissues can be exploited to improve the treatment procedure. It is well known that different body tissues have widely varying values of resistivity and dielectric constant. The resistivity of bone and fat is so high in relation to that of well-perfused tissues that one may consider bone and fat as electrical insulators. Thus, fat may have a resistivity of 1,000 to 3,000 ohm cm, compared to 200 ohm cm for typical muscle tissue. These variations in resistivity, while sometimes troublesome in forming treatment fields in connection with the present invention, may often be used to advantage. For example, the electrical current easily flows through a well perfused tumor volume immediately adjacent to the spinal column, while making only comparatively slight penetration of the spinal cord itself. This is due to the fact that the spinal cord is surrounded by the relatively high-resistance bone structure. Thus, the tumor volume could be heated to a level sufficient for radio sensitation, i.e., 42.degree. to 43.degree. C while the spinal cord remains essentially unheated with normal radiation sensitivity.